The present invention relates to thermoplastic elastomer compositions made from blends of 1-olefin polymers such as polypropylene, styrene-butadiene rubber, and a triblock copolymer such as styrene-butadiene-styrene or a diblock copolymer such as styrene-butadiene or a polystyrene polymer, with the compositions requiring no curing or vulcanization to develop elastomeric properties. Additionally, the invention also relates to partially cured compositions.
Heretofore, a few specific types of thermoplastic elastomers have been known. The term "thermoplastic elastomer" has generally been applied to elastomers which can be readily processed and reprocessed, molded, or the like by common or conventional thermoplastic methods and which do not require vulcanization to develop the various physical properties.
A recently developed type of thermoplastic elastomers is a blend of polypropylene and EPDM (ethylene-propylene-nonconjugated diene monomer), as described in U.S. Pat. Nos. 3,758,643; 3,806,558; and 3,862,106 to Fischer of Uniroyal, Inc. These blends are partially cured and contain EPDM in amounts up to 90 percent by weight. It is not surprising that blends of EPDM and polypropylene form a material having good mechanical properties since, due to the fact that EPDM contains a large number of monomer units in its backbone identical to those in polypropylene, there is good compatibility between these two polymers.
Various prior art patents apparently disclose blends of 1-olefin polymers and rubbers. However, these patents fail to suggest a blend containing a triblock copolymer which imparts superior physical properties, or of partially curing such blends, with or without the triblock copolymer. For example, U.S. Pat. No. 3,572,721 to Harrison relates to a golf ball, made from blends of rubber and a thermoplastic. In order to render the ball effective for use, it must be cured.
U.S. Pat. No. 3,665,059 to Mahlman relates to polyphase compositions having, as a continuous phase, a propylene polymer and, as a second phase, a synthetic or natural rubber which are prepared by dispersing a propylene polymer of substantially submicron particle size in the second phase and by fusing the polymer under conditions which assure the formation of the propylene polymer in a continuous phase.
U.S. Pat. No. 3,701,702 to Schichman relates to a method of free curing rubber, whereby the vulcanizate is obtainable by adding to natural rubbers, synthetic rubbers or blends thereof a fiber-forming thermoplastic resin.
The prior art also includes numerous patents which generally relate to thermoplastic compositions made from blends of a triblock copolymer and an olefin polymer. Typical examples include U.S. Pat. Nos. 3,299,174; 3,424,649; 3,689,595; 3,793,283; 3,830,767; and 3,850,474. However, such polymers lack any suggestion of a tricomponent blend containing a styrene-butadiene rubber, as well as the suggestion of partially curing the composition.
U.S. Pat. No. 3,576,911 relates to a polymeric material used for shoe soling which is prepared by blending various polymers. This patent fails to teach the use of a 1-olefin compound as well as any partial curing.
U.S. Pat. No. 4,017,436 basically relates to a thermoplastic resin composition containing a liquid butadiene-styrene copolymer of low molecular weight, 300 to 5,000, and a tackifier. Thus, this patent also lacks the polypropylene polymer as well as any suggestion of a partial cure.